When Alexandra Allen was a little girl, she wanted to be a marine biologist and live on a sailboat. After being diagnosed with an allergy to water, the 17-year-old from Mapleton, Utah, said she realizes that dream isn't likely to come true.

Allen said she had her first severe reaction to water when she was about 12. While on vacation with her family, she went swimming in a hotel pool and later that night woke up itching and covered in hives, she recalled.

"I remember sitting in the bathroom trying so hard not to scratch myself and make it worse until my mom came back with the Benadryl," the high school senior told ABC News.

She said she assumed at first that she was allergic to chlorine or some other harsh chemical, so she avoided swimming pools. But she knew the problem was much larger when she broke out into hives after swimming in a lake known for having very clean water.

When Allen was about 15 she came across a medical site that highlighted aquagenic urticarial, a condition defined by a painful reaction from skin contact with water as well as dry skin and dry eyes, she said, noting that it described her symptoms perfectly. And when she took it to her dermatologist, he agreed.

Alexandra Allen

Alexandra Allen, a 17-year-old from Mapleton, Utah, has been diagnosed with an allergy to water.

"He brought in a few other doctors and they just sat around in awe," she recalled, adding that the test to confirm the diagnosis, which involved soaking in a tub of water, felt "like being tortured."

Aquagenic urticarial is so rare that only about 50 cases have been described in medical literature, said Dr. Barney J. Kenet, a dermatologist with the Cornell Medical Center.

"It's a real thing. We learn about it in medical school, though I have never seen a case in my practice," Kenet said.

While not a true allergy, it causes severe allergy-like reactions, even after exposure to rain, snow, sweat or tears, according to an article in the Journal of Allergy Immunological Practice, one of the few studies to describe the disease. It tends to affect women more than men and usually first appears during puberty.

The cause of aquagenic urticarial is not well understood, Kenet said. One theory is that the sweat glands within the skin produce a toxin that triggers the allergic response, he said. Or it could be that antigens that cause the immune system to produce antibodies are absorbed in the skin after dissolving in water to trigger the allergic reaction.

Finding ways to avoid water has definitely been a challenge, Allen said. Obviously swimming is out. She has become a vegetarian to reduce the oils in her skin, avoids sweating and can only take two to three very short, cold showers a week, she said. Even humid climates can bring on a reaction, as she found out last year during a trip to Cambodia with a humanitarian aid group.

Her condition is thought to be degenerative, meaning that it gets worse with time and repeated exposures, Allen said. She expects at some point that drinking water may become a problem. Last year, she spoke to a British woman with the same diagnosis who told her she can now only drink Diet Coke.

But Allen said she remains positive. She tries to focus on the upside of her situation.

"At least I'm not allergic to dogs -- and it does get me out of doing the dishes," she said.